A conventional cutting insert having a substantially circular shape in a plan view is known. An example of such cutting insert is disclosed in Patent Document 1. The cutting insert of Patent Document 1 is a so-called positive type cutting insert comprised of a substantially circular upper surface, a lower surface (bottom surface) arranged so as to be opposed to the upper surface, and a peripheral side surface (side surface) connecting the upper surface and the lower surface. An intersecting edge between the upper surface and the side surface is provided with a circular-arc-shaped cutting edge throughout the intersecting edge. Further, the peripheral side surface is provided with a plurality of recessed parts at certain distances in a circumferential direction of the cutting insert. An engagement part (positioning part) formed by such recessed parts is engaged with an insert seat (support seat), whereby the cutting insert is firmly fixed to the insert seat. This also provides the effect of facilitating the indexing of a cutting edge portion involved in cutting.
A rotary cutting tool having a plurality of such substantially circular cutting inserts mounted thereon is suited for various kinds of machining for forming three-dimensional shapes. Further, the cutting insert is rotated so that cutting edge portions which are to be involved in cutting may be sequentially replaced with one another so as to be used, which is economical.